The present study aimed to determine whether certain

correction factors used in the in-place prediction ofcompressive strength with concrete cores are directlyapplicable to self-consolidating concretes (SCCs). Theparameters considered were core diameter, casting direction, core moisture, a number of variables intrinsic tocores, and concrete strength.

The findings show that the correction factors recommended in EHE-08 and ACI 214.4R-10 overestimate thein-place compressive strength of the SCC analysed. Thefactors found for converting cubic into cylindrical specimen strength, in turn, were observed to differ from thevalues set out in the 2010 Model Code, but to be similarto the EHE-08 code proposals.

Core testing is the most direct method for determining

the compressive strength of in-place concrete structures(1). The analysis and interpretation of core failure in traditionally vibrated concrete (CVC) have been the objectof many a study. Nonetheless, neither the suitability ofthis procedure nor the interpretation of the results hasbeen sufficiently studied for other types of concrete, selfconsolidating concrete (SCC) in particular.

For the intents and purposes of predicting in-place concrete compressive strength from core strength, Americanstandard ACI-214.4R-10 (1) defines a series of correctionfactors, inter-related as shown below [1]:

whereis equivalent in-place strength, fc,core is corestrength, Fl/d is the correction factor for the core length/diameter ratio, Ff is the correction factor for the corediameter, Fmoist is the correction factor for the coremoisture content and Fd is the correction factor for damage due to drilling. The mean values for these factors aregiven in Table 1.

Spanish structural concrete code EHE-08 (2) defines no

procedure for predicting cast specimen strength fromcore strength. The code only notes, in the commentary,that core strength is affected by drilling, in which theappearance of microcracks may induce a certain amount

The present study adopted the multiplicative approach

described in ACI-214.4R-10, in which the variablesstudied were core diameter, casting direction and coremoisture condition, along with concrete strength andcertain intrinsic properties that are difficult to measuredirectly (possible microcracking during drilling, pop-outof damaged coarse aggregate particles during testing,wall or boundary effects and so on). Further informationon these intrinsic variables can be found in Prez (3).

To find the response variable, i.e., in-place compressive

strength, the specimens were cast under conditions asclose as possible to the conditions used for in-place elements (SCC blocks). To that end, 15x30cm cylindricalspecimens were cast from the same batch of concreteas the SCC blocks and under similar conditions: i.e., withno mechanical consolidation and cured in worksite conditions. In this study the compressive strength found forthese cylindrical specimens is called equivalent in-placestrength and symbolised as fcyl,inplace.

Cubic and cylindrical specimens are the two types of

samples most commonly used to determine concretecompressive strength. Fifteen-centimetre cubic specimens are used in some European countries, includingthe United Kingdom and Germany, while in the UnitedStates, South Korea, France, Canada and Australia thestandard specimen is a 15x30cm cylinder (4). TheSpanish structural concrete code, Code EHE-08, makesprovision for both types of specimens.

The SCC for this study consisted of the standard material

manufactured by a Spanish precasting company (batching details in Table 2) using I52.5R portland cement andcoarse aggregate with a top size of 12mm. The characteristics of the concrete mixtures were: strength from 20to 80MPa; slump flow test of 665 to 750mm; and L-boxblocking ratios of 0.83 to 0.95.

The SCC mixes were used to cast thirty 50x50x100cm

blocks. Twenty-two days after casting, twelve sampleswere cored from each block, six perpendicular (horizontal) and six parallel (vertical) to the direction of casting.The cores were 5, 7.5 or 10cm in diameter and had aslenderness ratio of 2. Half of the total 360 cores drilledwere soaked in water for 48 hours and the rest wereallowed to dry under laboratory conditions. Both endsof the cores were capped with sulfur mortar to ensurethat they would be both perfectly flat and parallel to oneanother.

To simulate concrete with high steel ratios, in 14 of the

30 blocks the SCC was poured onto a lattice at the uppermost part of the forms, consisting of 12mm diameterbars spaced at 20 mm (Figure 1). The blocks and castspecimens were cured under identical conditions thatwere similar to standard worksite specifications.

The information on the ratio between cylindrical specimen and core sample strength in CVC used in this studywas drawn from the authors PhD. thesis (Rojas (7)), adatabase containing 44 records reported by other authors using specimens similar to the ones tested.

Table 3 gives the compressive strength values for the

cores, classified by moisture condition, casting directionand core diameter. Table 4, in turn, lists the compressivestrength findings for the (cylindrical and cubic) specimens cast with the same concrete as each SCC block.

The strength found for the 15x30cm cylindrical specimens was regarded as closest to the in-place compressive strength, inasmuch as the SCC specimens and theirrespective blocks were manufactured and stored underconditions as similar as possible. The assumption was,then [2]:

fc,cyl = fcyl,inplace

[2]

donde fc,cyl es la resistencia a compresin de la probeta

cilndrica y fcyl,inplace es la resistencia equivalente in situ.

where fc,cyl is the compressive strength of the cylindrical specimen and fcyl,inplace the equivalent in-placestrength.

Materiales 312 (tripas).indd 504

Correction factors to predict the in-place compressive strength of a self-compacting concrete

Equation [4] was consequently valid for predicting the

fc,core / fcyl,inplace values for all the sub-samples studiedfrom the data on strength, diameter and moisture condition. Figure 2 shows the six regression lines obtainedwith the model, for the following Equations [5-10]:

The deductive procedure followed was as laid down in

Spanish and European standard UNE-EN 13791:2009.According to that standard, the strength of a core witha nominal diameter of 10 to 15cm and a slendernessratio of 2 is equivalent to the strength of a cylindricalspecimen prepared and cured under the same conditions. A dry-cured 10 x 20cm core was consequentlychosen as the reference core. Standard ACI-214.4R-10establishes this type of core as the reference forcalculating correction factors. Figure 3 describes theprocedure used to calculate the correction factors forthe SCC analysed.

The three factors were observed to vary with concrete

strength. That finding is not consistent with standardACI-214.4R-10, which lays down only one value for eachcorrection factor, irrespective of concrete strength. Thecorrection factor proposed by the ACI standard for 5cmdiameter cores, 1.06, concurs with the Ff found in thisstudy for SCCs in the 6070MPa strength range only.

Concrete strength also proved to affect the difference

in dry and moist core strength, and therefore the coremoisture correction factor, Fmoist. That effect may be related to the variations in permeability between low- andhigh-strength concretes. The correction factor for moistcores, Fmoist proposed in ACI-214.4R-10 is 1.09 for CVCand from 1.12 to 1.19 for SCC, depending on strength.

standards and by other authors for CVCs. As described

earlier, correction factor Fbasic was obtained by comparingthe strength of 15x30cm cylindrical specimens to thestrength of 10x20cm cores made from similarly consolidated and conditioned (dry in both cases) concrete.In other words, this comparison indirectly assessed theother variables (intrinsic features such as possiblemicrocracking due to drilling, or the wall and boundaryeffects) that are difficult to measure directly.

Figure 4 shows the Fbasic values observed for SCC as tiered

horizontal lines obtained by transforming the second-degree line in step 4 of the deductive procedure. In addition,the figure shows the curve fitted to the data from otherstudies that assessed the fcyl,inplace / fc,core ratio in CVCelements with specifications similar to the characteristicsof the present materials -drawn from the compilation inRojas PhD. thesis (7). The constant values for the Fbasicfactor specified in Spanish code EHE-08 and Americanstandard ACI 214.4R-10 are likewise shown.

The obvious difference in the equivalent in-place strength

and core strength ratios between the two types of concrete is a clear indication that the Fbasic correction factors proposed for CVC should not be used for the SCCstudied, for they would overestimate equivalent in-placestrength, particularly for lower strengths.

Figure 4 also shows that the equivalent in-place

strength / core strength ratios for CVC are higher than thevalues recommended in code EHE-08 and standard ACI214.4R-10, i.e., 1.11 and 1.06, respectively. The valuesset out in these standards are on the safety side, particularly for high-strength concrete.

Two 5cm cores with a length/diameter ratio of 2 were

drilled to estimate the strength of a self-consolidated, inplace concrete column, assuming consistently dry serviceconditions. One of the cores was drilled in the uppermostand the other in the lower part of the column. The coreswere then soaked for 48hours and subsequently testedto failure. Their compressive strength was found to be32.8and 33.2MPa.

These findings were in keeping with the reports of other

authors working with CVC. Mansur and Islam (10), forinstance, who obtained a curve with a slope nearly equalto 1.0, concluded that the difference between cubic andcylindrical specimen strength was practically constant irrespective of concrete strength.

Figure 5 plots the cylindrical compressive strength

against the cubic compressive strength of the SCCanalysed that resembled the Model Code (11) valuesfor CVC most closely. The 45 reference line is alsorepresented. The figure shows that the CVC and SCCregression lines tend to pass very close to the origin.In other words, Fc,cub rose in practically constant proportion to fc,cyl. Moreover, the characteristics strenghtfor cylindrical and cubic specimens are closer in SCCthan in CVC. Due to self-consolidation, the paste inSCC is more uniform. Consequently, it is possible that

Several correction factors were determined for predicting in-place SCC strength from core strength, namelyfor core diameter, casting direction, core moisture anda series of jointly considered intrinsic variables such asmicrocracking and the wall and boundary effects. Thevalues obtained for these factors were observed todepend on concrete strength.

The two types of concrete studied (SCC and CVC)

exhibited differences in the equivalent in-placestrength and core strength ratio. In SCC that ratio isnot constant, but varies with concrete strength. Thevalues laid down in Spanish code EHE-08 and ACI214.4R for CVC (1.11 and 1.06, respectively) are notapplicable to the SCC studied here, for they wouldoverestimate the equivalent in-place strength.

The differences between the two types of concretes may be essentially attributed to the degree ofconsolidation. Namely when performing correlationsbetween molded cylinders and cores, compaction ismore similar in samples of SCC that CVC. Factors for converting cubic to cylindrical specimenstrength in SCC were also calculated in this study.These factors varied scantly for cubic specimens in thestrength range studied (30 to 80 MPa). The factorsfound 0.90 to 0.93, were similar to the EHE-08 proposals but differed from the 2010 Model Code values.